The present invention relates to a superconductive conductor or cable comprising a core, which comprises at least one string of a ceramic superconductive material, and where the core is encapsulated by a metallic cap. Furthermore the invention relates to a method of manufacturing such a conductor or cable.
Ceramic compounds of the type, which are often designated by, the formula Y-Ba-Cu-O have metals in oxidized form and exhibit electric superconductive characteristics at a substantially higher temperature than that of conventional superconducting materials. Because of the relatively high marginal temperature of the superconductive characteristics, this type of compounds has become more interesting for use in a greater variety of industrial application. The ceramic compounds form a whole group of materials, in which copper and oxygen seem to be the only required materials, as they, in addition to or instead of the metals yttrium and barium, may also comprise e.g. scandium, various lanthanides, calcium or strontium. Superconductors comprising other materials, e.g. bismuth and/or thallium are also known, as such materials may completely or partly replace the abovementioned metals. Furthermore, it is known that the superconductive characteristics may be improved in some cases, if fluorine replaces part of the oxygen.
The use of the superconductive materials is restricted by the fact that the manufacture of these conductors in sufficient lengths and with a sufficiently homogeneous structure for the industrial application of fabricating cables is difficult and costly. An essential problem in this connection is that in the manufacturing process a solid-reaction between suitable compounds, e.g. oxides, carbonates or oxalates must take place; that the reaction must take place under a controlled set of temperature, and that the composition of the surrounding atmosphere may be of considerable importance. In particular, a high partial pressure of oxygen may be necessary. Furthermore there is a considerable risk that the reaction product thus formed may be unstable and liable to split off e.g. oxygen. It may, therefore, be necessary to surround the superconductive core, also under later operation, by a controlled atmosphere.
These problems are discussed in Fujikura Technical Review, No. 17, Feb. (Tokyo JP), H. Osani et al, pages 1-4, in which different materials for the metal cap have been tested, and in Advanced Ceramic Materials--Ceramic Superconductors, Volume 2, No. 38, Special Issue, Jul. 1988, (Westerville, Oh., US), R. W. McCallum et al pages 388-400, in which silver is identified as a preferred material for encapsulation because of its permeability to O.sub.2, and in which addition of silver oxides inside the cap has been suggested as a means for controlling the atmosphere inside the cap during the sintering process of the superconductive material.